You should use $a[EXPR] when you mean to access a single element in order to convey this information to the reader. In fact, you can get a warning if you don't.

You should use @a[LIST] when you mean to access many elements or a variable number of elements.

But that's not the end of the story. You asked for practical and tricky (subtle?) differences, and there's one noone mentioned yet: The index expression for an array element is evaluated in scalar context, while the index expression for an array slice is evaluated in list context.

The @ sigil means "give me a list of something." When used with an array subscript, it retrieves a slice of the array. For example, @foo[0..3] retrieves the first four items in the array @foo.

When you write @a[1], you're asking for a one-element slice from @a. That's perfectly OK, but it's much clearer to ask for a single value, $a[1], instead. So much so that Perl will warn you if you do it the first way.